1. Field of the Invention
The present invention relates to an optical pulse signal generating apparatus optical-time-division-multiplexing (OTDM) an optical pulse signal that is coded by a differential phase shift keying (DPSK) system, so as to generate an optical-time-division-multiplexing differential phase shift keying signal (OTDM-DPSK signal), and outputs the OTDM-DPSK signal. In particular, the present invention relates to optical carrier phase difference detection between adjacent optical pulses of an OTDM-DPSK signal.
2. Description of the Related Art
In recent years, in the technical field of optical communication, the balanced detection system has been studied in addition to the intensity modulation-direct detection (IM-DD) system (refer to, for example, R. Ludwig et al., “160 Gbit/s DPSK-Transmission—Technologies and System Impact”, Proc. 30th European Conference on Optical Communication (ECOC 2004), Tul. 1, 3: document 1).
The IM-DD system is a system that carries out detection by detecting the envelope of intensity of the optical carrier of a received signal by using a photodiode. The IM-DD system is a representative example of optical communication by an optical pulse signal that is coded by the ASK (Amplitude Shift Keying) system. Intensity modulation is generally referred to as ASK or OOK (On Off Keying). Note that, intensity modulation is referred to as “ASK” in the following description.
The balanced detection system is a system that carries out detection by detecting an electrical signal having an amplitude that is twice the magnitude as in the IM-DD system, by using a balanced detector. The balanced detection system is a representative example of optical communication by an optical pulse signal that is coded in the DPSK format.
In the IM-DD system, receiving and transmitting are possible by a simple device, and the IM-DD system can also be applied to an intensity reproducing device in which an optical amplifier is built-in. Due thereto, the IM-DD system is widely used. On the other hand, in the DPSK system, modulation of an optical pulse train is carried out by the two variables of 0 and π, and a transmission signal is generated and transmitted. At the receiving end, the received signal is divided in two into first and second received signals, and a time delay of the time period that one optical pulse (one bit) occupies on the time axis is provided to the first received signal. Then, the receiving end combines the first received signal, to which the time delay has been provided, and the second received signal, and carries out balanced detection on the received signal obtained by combining these signals. Hereinafter, 1/n of the time period that one optical pulse occupies on the time axis, where n is a positive integer, is called the “time corresponding to 1/n bit”. Further, the transmission signal (optical pulse signal) that is generated by carrying out modulation of the optical pulse train by the two variables of 0 and π is called the “transmission signal coded in the DPSK format” or the “optical pulse signal coded in the DPSK format”.
The optical pulses and the phase of the optical pulses will be described here. An optical pulse that is observed as a change in optical intensity is expressed by the envelope of the waveform of the square of the amplitude of the electrical field vector of the optical carrier. Accordingly, in the following description, “time waveform of the optical pulses” refers to the time waveform of the envelope of the square of the amplitude of the electrical field vector of the optical pulses.
The phase of the optical carrier of the optical pulses means the relative phases of the peak of the optical carrier with respect to the peak of the envelope of the electrical field vector of the optical pulses. The phase of the optical carrier of the optical pulses is called the “optical carrier phase” or the “absolute phase”. Further, the optical carrier phases or the absolute phases are more strictly called the “carrier-envelope phases” (often abbreviated as CEP). Hereinafter, the phases of the optical carrier of the optical pulses is called the “optical carrier phases”. The envelope of the electrical field vector of one optical pulse includes a very large number of peaks of the optical carrier.
For example, in a case in which the wavelength of the optical carrier is 1.5 μm, when converted into frequency, it becomes about 2×1014 Hz. On the other hand, when the repetition frequency of the optical pulses is 40 GHz, it is around 4×10 Hz. Accordingly, in this case, 5000 (=(2×1014)/(4×1010)=5×103) peaks of the optical carrier are included in the envelope of the electrical field vector of one optical pulse, i.e., one optical pulse in the time waveform.
The aforementioned “modulating of optical pulses by the two variables of 0 and π” means leaving the phase of the electrical field vector of the optical carrier that forms the optical pulse (the phase of the optical carrier) as is or shifting it by π phase, with respect to the envelope of the optical carrier. Namely, “modulating of optical pulses by the two variables of 0 and π” means leaving the optical carrier phase as is or shifting it by π phase with respect to the envelope of the electrical field vector of the optical pulse.
In a case of using the balanced detection system applied to the DPSK system, the receiver sensitivity is improved by greater than or equal to 3 dB, as compared with the IM-DD system applied to the ASK system (refer to document 1 for example).
In the OTDM communication system of the DPSK system disclosed in document 1, a 40 Gbit/s DPSK signal is generated, and this DPSK signal is divided into four. Next, other than the signal of the first optical path (the signal of one channel), time delays corresponding to ¼, 2/4, ¾ bits are provided to the respective signals of the three optical paths that are the second through fourth optical paths (signals of three channels). Further, the signals of the first through fourth optical paths, including the signal of the first optical path to which a time delay was not provided, are multiplexed. In this way, an OTDM signal is generated. Namely, among the signals of the four optical paths, the signal of the first optical path is the actual signal. However, among the signals of the four optical paths, the signals of the remaining three optical paths that are the second through fourth optical paths are copy signals of the signal of the first optical path. A practical generating apparatus must use respectively different signals for the signals of the first through the fourth optical paths respectively. Accordingly, in the case where the OTDM communication system of the DPSK system disclosed in document 1 is used as is, an apparatus that generates practical OTDM-DPSK signals cannot be practically realized.
As an apparatus OTD-multiplexing a practical, multiple-channel optical pulse signal that is coded in the DPSK format, and generates an optical-time-division-multiplexing differential phase shift keying signal (hereinafter called an OTDM-DPSK signal generating apparatus), an example that is structured on the basis of the system disclosed in document 1 will be explained with reference to FIG. 1. FIG. 1 is a schematic block structural diagram of an OTDM-DPSK signal generating apparatus.
An OTDM-DPSK signal generating apparatus 10 is configured to include: an optical splitter 12; a first phase modulator 14; a second phase modulator 16; a ½-bit delay device 18; and an optical coupler 20. Transmission signals are supplied to the OTDM-DPSK signal generating apparatus 10 from a first modulator driver 22 and a second modulator driver 24.
An optical pulse train 11, at which optical pulses are lined-up at uniform intervals on the time axis, is inputted to the OTDM-DPSK signal generating apparatus 10. The optical pulse train 11 is divided in two at the optical splitter 12 and generated as first optical pulse train 13-1 and second optical pulse train 13-2. The first optical pulse train 13-1 and the second optical pulse train 13-2 are respectively inputted to the first phase modulator 14 and the second phase modulator 16.
At the first phase modulator 14 and the second phase modulator 16, the first optical pulse train 13-1 and the second optical pulse train 13-2 are respectively coded in the DPSK format by transmission signals 23 and 25 that are supplied from the first modulator driver 22 and the second modulator driver 24. The coded first optical pulse train 13-1 and the second optical pulse train 13-2 are generated as a first differential phase shift keying signal 15 and a second differential phase shift keying signal 17, and are outputted respectively.
The second differential phase shift keying signal 17 is inputted to the ½-bit delay device 18. A time delay corresponding to ½ bit is provided to the inputted second differential phase shift keying signal 17, and it is generated as a delayed second differential phase shift keying signal 19 and outputted. The first differential phase shift keying signal 15 and the delayed second differential phase shift keying signal 19 are multiplexed at the optical coupler 20, and generated as an OTDM-DPSK signal 21 and outputted. Namely, on the basis of the optical pulse train 11, the OTDM-DPSK signal generating apparatus 10 converts the transmission signals of the two channels, that are supplied from the first modulator driver 22 and the second modulator driver 24, into DPSK signals. Then, the OTDM-DPSK signal generating apparatus 10 OTD-multiplexes these DPSK signals of the two channels, and outputs the OTDM-DPSK signal.
FIG. 1 shows an example of realizing OTDM of two channels. The situation is the same as in FIG. 1 no matter how many channels there are, provided that the number of channels is a number given by 2N (where N is an integer of greater than or equal than 1). For example, in the case of realizing OTDM of four channels, it suffices to provide time delays corresponding to 0, ¼, 2/4, ¾ bits respectively to the optical pulse signals of the first through fourth channels that are coded in the DPSK format, and to multiplex the signals.
In order for the OTDM-DPSK signal generating apparatus 10 to operate, it is necessary for the first optical pulse train 13-1 and the second optical pulse train 13-2 that are branched by the optical splitter 12 to not receive phase modulation other than the phase modulation provided by the first phase modulator 14, the second phase modulator 16 and the ½-bit delay device 18 respectively. Namely, at the optical coupler 20, the optical carrier phase difference between the optical pulses that structure the first differential phase shift keying signal 15 and the optical carrier phase difference between the optical pulses that structure the delayed second differential phase shift keying signal 19 must not take-on values other than 0 or π.
However, at the optical paths that the first optical pulse train 13-1 and the second optical pulse train 13-2 propagate through, and the optical paths that the first differential phase shift keying signal 15 and the second differential phase shift keying signal 17 propagate through, and the optical path that the delayed second differential phase shift keying signal 19 propagates through, fluctuations in the optical path length arise due to variations in temperature and the like. When comparing the magnitude of the fluctuations in the optical path length and 0 or π as converted into optical carrier phases, it is very difficult technically to keep the magnitude of the fluctuations in the optical path length to an extent such that they can be ignored; Accordingly, there is the need to, by some method, detect and control the optical carrier phase difference between adjacent optical pulses that structure an optical pulse signal.
There are known methods of detecting and controlling the optical carrier phase difference (see, for example, Japanese Patent Applications Laid-Open (JP-A) Nos. 2005-006175 and 2007-189616). The method of detecting the optical carrier phase difference disclosed in JP-A No. 2005-006175 branches-off a portion of an optical phase signal that is coded by the ASK system. Then, this method leads the branched-off optical pulse signal to an interferometer, and observes the intensity of the interference light outputted from the interferometer. The intensity value of the interference light is used as a control signal. The method of detecting the optical carrier phase difference disclosed in JP-A No. 2007-189616 leads, to an interferometer, an ASK time division multiplex signal that is generated by time division multiplexing an optical pulse signal coded by the ASK system, and converts the signal outputted from the interferometer into an electrical signal. This method observes the optical carrier phase difference by an electrical circuit observing the time average value of the electrical signal. This time average value is detected as the optical carrier phase difference, and is used as a control signal.
Other than the above-described methods of optical carrier phase difference detection with respect to an ASK time division multiplex signal obtained by time division multiplexing an optical pulse signal coded by the ASK system, there are also known optical carrier phase difference detection methods with respect to an OTDM-DPSK signal that is generated by an optical pulse signal, that is coded in the DPSK format, being optical-time-division-multiplexed (refer to JP-A No. 2008-085889). Interference signal light, that has been intensity-modulated in accordance with the optical carrier phase difference between the optical pulses by an optical interferometer that is specially devised and to which OTDM-DPSK signals are inputted, is converted into an electrical signal, and the time average value of the electrical signal is observed by an electrical circuit. This time average value is detected as the optical carrier phase difference and is utilized as a control signal.
With regard to phase fluctuations of the optical carrier phase difference between adjacent optical pulses that are detected by optical carrier phase difference detection, here, an optical pulse signal of the carrier-suppressed-RZ (CS-RZ) format will be described as an example. A CS-RZ format optical pulse signal is an optical pulse signal generated by coding, by the ASK system, an optical pulse train in which adjacent optical pulses are lined-up with the optical carrier phases thereof having a phase difference of π (hereinafter called “CS optical pulse train” upon occasion).
In order to split a portion of an OTDM format optical pulse signal and detect the optical carrier phase difference, the optical pulse signal is introduced into an optical carrier phase difference detecting apparatus. A CS-RZ format optical pulse signal inputted to the optical carrier phase difference detecting apparatus is divided in two, a delay of one bit is provided to one, and the both are again made to interfere. When an interference signal is outputted from the optical carrier phase difference detecting apparatus in this way, the optical carrier difference between adjacent optical pulses does not take-on a value other than π. In the case of an ideal optical pulse signal in the CS-RZ format, the intensity of the output light is 0.
The CS-RZ format is generated from the optical pulses whose phases are inverted at 0, π at each one bit. Therefore, if an optical pulse train is divided in two and a one-bit delay is provided to one and they are made to interfere, adjacent optical pulses whose optical carrier phase difference is π interfere with one another.
On the other hand, in a case in which the optical carrier pulse difference between adjacent optical pulses of a CS-RZ format optical pulse signal takes-on a value other than 0 or π, an interference signal whose intensity is a value greater than 0 is outputted from the optical carrier phase difference detecting apparatus. Namely, in a case in which the optical carrier phase difference between the adjacent optical pulses is offset from 0 or π by φ (0≦φ≦π), the intensity of the outputted interference signal becomes greater as the value of φ approaches 0, and becomes the maximum intensity when φ=0. Accordingly, the value of φ can be known by monitoring the time average value of the intensity of the interference signal outputted from the optical carrier phase difference detecting apparatus. Further, in a case in which feedback control is carried out so that the time average value of the intensity of the outputted interference signal always becomes the minimum, an ideal CS-RZ format optical pulse signal, in which the optical carrier phase difference between adjacent optical pulses does not take-on a value other than π, can be generated. Hereinafter, φ is called the “magnitude of the phase fluctuation”.
However, the above-described optical carrier phase difference detecting methods that use optical pulse signals in the CS-RZ format cannot use OTDM-DPSK signals that are outputted from the OTDM-DPSK signal generating apparatus described with reference to FIG. 1. The reasons therefore will be described with reference to (1), (2) and (3) of FIG. 2. (1) of FIG. 2 shows the time waveform of an OTDM-DPSK signal 21 at the OTDM-DPSK signal generating apparatus shown in FIG. 1. (2) of FIG. 2 shows the time waveform of a signal which is achieved by splitting the OTDM-DPSK signal 21 within the interferometer for detecting the optical carrier phase difference disclosed in JP-A No. 2005-006175, and applying a delay of 1 bit. Further, (3) of FIG. 2 shows the time waveform of a signal which is re-coupled within the interferometer and outputted therefrom. The time waveforms shown in (1), (2) and (3) of FIG. 2 show, among the envelopes of the amplitude waveforms of the electrical field vectors of the optical carriers, the envelopes at the sides that take-on positive values, and the envelopes at the sides that take-on negative values are deleted. Further, the time axis is shown at an arbitrary scale in the horizontal axis direction, and the magnitude of the amplitude is shown at an arbitrary scale in the vertical axis direction.
The optical carrier phases of the optical pulses respectively structuring the first OTDM-DPSK signal 21 shown in (1) of FIG. 2 and the delayed second OTDM-DPSK signal shown in (2), interfere at phase differences of 0 and φ, π and φ, 0 and (π+φ), and π and (π+φ). If the first OTDM-DPSK signal 21 is generated as an ideal OTDM-DPSK signal, the magnitude of the phase fluctuation is 0, i.e., φ=0. However, as described above, fluctuations in the optical path lengths arise at the optical paths through which the optical signals propagate. Therefore, the magnitude of the phase fluctuation cannot always maintain the state φ=0 unless some type of control is carried out.
At the first phase modulator 14 and the second phase modulator 16, modulation of 0 or π as the optical carrier phase is carried out on the optical pulses. Namely, either modulation that does not change the optical carrier phase is carried out (modulation of 0 is carried out), or modulation that shifts the optical carrier phase by π is carried out (modulation of π is carried out) on the optical pulses.
As a result of the modulation at the first phase modulator 14 and the second phase modulator 16, either phase modulation in which the optical carrier phase is maintained as is (shown as 0 in (1) and (2) of FIG. 2) or phase modulation in which the optical carrier phase is shifted by π (shown by π in (1) and (2) of FIG. 2) is applied to the first OTDM-DPSK signal 21. There exist optical pulses to which a phase fluctuation of φ (shown by φ in (1) and (2) of FIG. 2), that is due to fluctuations in the optical path lengths and the like, is applied to each of the optical carrier phases of the optical pulses structuring the first OTDM-DPSK signal 21, in addition to this phase modulation.
The time waveform of an interferometer output signal that is generated as a result of combining the first OTDM-DPSK signal 21 shown in (1) of FIG. 2 and the delayed second OTDM-DPSK signal shown in (2), is the shape shown in (3). The reasons for this are as follows.
In (1), (2) and (3) of FIG. 2, the time widths of each one optical pulse are illustrated as being sectioned by the vertical broken lines. Attention is focused first onto the leftmost optical pulse in each of (1), (2) and (3) of FIG. 2. Among the optical pulses that structure the first differential phase shift keying signal 15 shown in (1) of FIG. 2, the optical carrier phase of the leftmost optical pulse is 0. Among the optical pulses that structure the delayed second differential phase shift keying signal 19 shown in (2), the optical carrier phase of the leftmost optical pulse is φ. In this case, the optical pulse that is generated as a result of interference of the both optical pulses has an amplitude of a finite magnitude (hereinafter called “amplitude of the first magnitude”) that is different than 0, as is the case with the leftmost optical pulse of an interferometer output signal shown in (3).
Similarly, at the second from the left, the optical pulse whose optical carrier phase is φ and the optical pulse whose optical carrier phase is π interfere. Therefore, an optical pulse having an amplitude (hereinafter called “amplitude of the second magnitude”) of a magnitude that is different than the aforementioned amplitude of the first magnitude is generated.
Looking in the same way as described above at the magnitudes of the amplitudes of the optical pulses shown in (1), (2) and (3) of FIG. 2 in order from the leftmost side, the following can be concluded. Namely, an optical pulse having the amplitude of the first magnitude is generated in cases in which the optical carrier phases of the optical pulses of the first OTDM-DPSK signal 21 and the delayed second OTDM-DPSK signal are the combination of 0 and φ, and the combination of π and (π+φ). Further, an optical pulse having the amplitude of the second magnitude is generated in cases in which the optical carrier phases of the optical pulses of the first OTDM-DPSK signal 21 and the delayed second OTDM-DPSK signal are the combination of φ and π, and the combination of 0 and (π+φ).
Accordingly, the optical pulses structuring the interferometer output signal are two types that have the first and the second magnitudes. The magnitudes of the amplitudes of the two types of optical pulses of the interferometer output signal are functions of φ that is caused by fluctuations in the optical path lengths and the like. Accordingly, if φ is near 0, the magnitude of the first amplitude is greater than the magnitude of the second amplitude. Further, if φ is near π, the magnitude of the second amplitude is greater than the magnitude of the first amplitude. (3) of FIG. 2 shows a case in which the magnitude of the first amplitude is greater than the magnitude of the second amplitude, i.e., a case in which φ is near 0.
When the value of φ changes in the range from 0 to π in this way, the magnitude of the first amplitude and the magnitude of the second amplitude vary on account of the relationship that, when one decreases, the other increases. Accordingly, the average intensity of the interferometer output signal does not fluctuate. Therefore, in the above-described optical carrier phase difference detecting methods, a phase difference φ between adjacent optical pulses of the OTDM-DPSK signal cannot be detected.
Further, in accordance with the apparatus for detecting the optical carrier phase difference between adjacent optical pulses of an OTDM-DPSK signal that is disclosed in JP-A No. 2008-085889, the magnitude φ of the above-described phase fluctuation can be detected. However, this apparatus requires an exclusive-use optical interferometer, and a combined optical coupler/splitter, and the like.